1. Field of the Invention
The present invention relates to an optical illumination system to be used mainly to illuminate a light modulator. The present invention also relates to a projection display apparatus for illuminating the light modulator and for projecting an enlargement of an optical image, which has been formed on the light modulator, onto a screen.
2. Description of the Prior Art
An optical illumination system using a light modulator is known as a means for displaying an image on a large screen. Specifically, a projection display apparatus using a liquid crystal panel has been developed in recent years. An example of such a projection display apparatus is described in SID' 91 Digest, page 415 through page 418.
It is necessary to provide such a projection display apparatus with an optical illumination system which illuminates an optical image formed on a light modulator with a strong light. The quality of the optical image projected onto the screen depends greatly on the performance of the optical illumination system. Accordingly, there is a growing demand for the development of an optical illumination system providing luminous flux which has good color reproducing property, good uniformity of brightness, and good uniformity of color, with high efficiency.
An optical illumination conventionally uses a concave mirror. FIG. 18 shows an example of this type of optical illumination system comprising a lamp 201 and a concave mirror 202. The concave mirror 202 consisting of a parabolic mirror condenses most of the beams of light radiating from a radiation source 203 and forms illuminating beams 205 substantially parallel to an optical axis 204, thus illuminating an object region 206.
A metal halide lamp, a xenon lamp or a halogen lamp is used as the lamp 201. The metal halide lamp is superior to the xenon lamp and the halogen lamp in luminous efficacy and color reproduction properties. The concave mirror 202 may be an ellipsoidal mirror.
In the optical illumination system using the concave mirror, the concave mirror condenses illuminating beams with a high efficiency, but since the luminous density in the vicinity of the optical axis 204 is high, the brightness of the image displayed at the object region 206 is nonuniform. The tube surface of the lamp 201 has been frosted to improve the uniformity of brightness and color of the image at the object region 206. However, a frosted tube diffuses beams and as a result, the object region 206 has a very low brightness.
An optical integrator has been used to improve the uniformity of brightness and color of illuminating light. Such an optical integrator, which uses lens arrays, is disclosed in examined Japanese Patent Publication No. 43-5089 and U.S. Pat. No. 5,098,184. FIG. 19 shows an example of an optical illumination system comprising the optical integrator.
The optical illumination system shown in FIG. 19 includes a first lens array plate 221, a second lens array plate 222, and a third lens 223 in addition to the components of the optical illumination system using the concave mirror as shown in FIG. 18. The first lens array plate 221 and the second lens array plate 222 comprise a plurality of first lenses 224 and a plurality of second lenses 225 arrayed two-dimensionally, respectively. The first lens array plate 221 divides a single luminous flux, having a great brightness nonuniformity, into the same number of partial luminous fluxes as the number of first lenses 224. The degree of the nonuniform brightness of each partial luminous flux is smaller than the degree of the nonuniform brightness of the single luminous flux. The partial luminous fluxes are effectively transmitted toward the object region 206 by the second lens array plate 222 and are superimposed on each other by the third lens 223. In this manner, illumination having a high degree of brightness uniformity can be obtained.
The conventional optical illumination system as shown in FIG. 19 has, however, a low efficiency of light in action when the illuminating angle of illuminating light is limited to a certain range although the optical illumination system can form an image having a high degree of brightness uniformity. The illuminating angle means the maximum of all of the angles formed between the beams, which illuminate the object region, with the main optical axis of the optical illumination system. This disadvantage is conspicuous if the optical illumination system has a large radiation source. The reason for this is as follows.
Most of the beams emitted by the lamp 201 are reflected by the concave mirror 202 and incide on the first lens array plate 221, thus reaching the second lens array plate 222. Accordingly, if there is no loss of light at the second lens array plate 222, most of the beams radiating from the lamp 201 reach the object region 206. That is, the efficiency of light in action of the entire illuminating optical system depends mainly on the optical loss at the second lens array plate 222.
A plurality of real images of the radiation source 203 are formed on the second lens array plate 222 via the concave mirror 202 and the first lens array plate 224. If a real image larger than the area of the aperture of the second lens 225 is formed thereon, some beams are not effectively transmitted to the object region 206 and an optical loss occurs. That is, the efficiency of light in action of the optical illumination system is lowered.
The size of the real image formed on the second lens array plate 222 is determined by the size of the radiation source 203. Therefore, preferably, a lamp comprising a small radiation source is used in view of the efficiency of light in action of the optical illumination system. However, in the case of a short arc type of metal halide lamp, the wattage of which is 150 W to 250 W, the length of the radiation source is as short as 5 mm to 10 mm. As a result, the emission characteristic of the lamp is extremely poor or the life thereof is short. The same is true with respect to a halogen lamp.
Only the second lens array plate could be enlarged to reduce optical loss without the need to otherwise alter the structure of the optical system. In this case, however, the illuminating angle would be large, and the optical illumination system would have the following problems.
A large illuminating angle requires the use of a projection lens having a small f-number. The projection lens has, however, a large effective diameter and is expensive. It is, therefore, difficult to provide a compact optical projection apparatus.
The optical characteristic of the conventional display changes according to the incident angle of the light. For example, the contrast of an image projected onto a screen from a liquid crystal panel display becomes poorer with increases in the illuminating angle. Therefore, the f-number of the projection lens which can be used is limited.